Technical Field
The present disclosure refers to a contact probe for a testing head.
The disclosure refers in particular, but not exclusively, to a contact probe of a testing head of a testing apparatus of electronic devices integrated on wafers and the following description is made with reference to this field of application with the only purpose of simplifying its exposition.
Description of the Related Art
As is known, a testing head (probe head) is a device adapted to place a plurality of contact pads of a microstructure, in particular an electronic device that is integrated on a wafer, into electrical contact with corresponding channels of a testing machine performing the working test thereof, in particular the electrical test, or generically the test.
The test, which is performed on integrated devices, is particularly useful to detect and isolate defective devices yet in the manufacturing step. Generally, the testing heads are thus used to electrically test the devices that are integrated on a wafer before cutting and assembling them inside a chip containing package.
A testing head usually comprises a large number of contact elements or contact probes made of wires of special alloys having good electrical and mechanical properties and provided with at least one contact portion for a corresponding plurality of contact pads of a device to be tested.
A testing head comprising vertical probes, which is usually called “vertical probe head”, comprises a plurality of contact probes retained by at least one pair of plates or guides, which are substantially plate-shaped and parallel to each other. Those guides are provided with suitable holes and are arranged at a certain distance from each other so as to leave a free space or air gap for the movement and the possible deformation of the contact probes. The pair of guides comprises in particular an upper guide and a lower guide, both provided with respective guide holes where the contact probes axially slide, the probes being usually made of wires of special alloys having good electrical and mechanical properties.
The good connection between the contact probes and the contact pads of the device to be tested is realized by pressing the testing head on the device itself, the contact probes, which are movable within the guide holes made in the upper and lower guides, undergoing a bending inside the air gap between the two guides and sliding within such guide holes during that pressing contact.
Furthermore, the bending of the contact probes in the air gap can be assisted by means of a suitable configuration of the probes themselves or of their guides, as schematically shown in FIG. 1, wherein, for sake of illustration simplicity, only one contact probe of the plurality of probes usually included in a testing head has been shown, the shown testing head being of the so-called shifted plates type.
In particular, FIG. 1 schematically shows a testing head 1 comprising at least one upper plate or guide 2 and one lower plate or guide 3, having respective upper guide hole 2A and lower guide hole 3A where at least one contact probe 4 slides.
The contact probe 4 has at least one contact end or contact tip 4A. Here and in the following, the terms end or tip mean an end portion, not necessarily being sharp. In particular the contact tip 4A abuts onto a contact pad 5A of a device to be tested 5, realizing the mechanical and electrical contact between that device and a testing apparatus (not shown) of which such testing head is a terminal element.
In some cases, the contact probes are fixedly fastened to the head itself in correspondence of the upper guide: in such case, the testing heads are referred to as blocked probe testing heads.
Alternatively, testing heads having probes not fixedly fastened are used, those probes being interfaced to a board by means of a micro-contact board; those testing heads are referred to as non-blocked probe testing heads. The micro-contact board is usually called “space transformer” since, besides contacting the probes, it also allows to spatially redistribute the contact pads made on it with respect to the contact pads of the device to be tested, in particular relaxing the distance constraints between the centers of the pads themselves.
In this case, as illustrated in FIG. 1, the contact probe 4 has a further contact tip 4B, which in this technical field is referred to as contact head, towards a plurality of contact pads 6A of such a space transformer 6. The good electrical contact between probes and space transformer is guaranteed in a similar manner to the contact with the device to be tested by pressing the contact heads 4B of the contact probes 4 against the contact pads 6A of the space transformer 6.
As already explained, the upper 2 and lower 3 guides are suitably spaced by an air gap 7 which allows the deformation of the contact probes 4 and ensures the contact of the contact tip and of the contact head of the contact probes 4 with the contact pads of the device to be tested 5 and of the space transformer 6, respectively. Obviously, the upper 2A and lower 3A guide holes are sized so as to allow a sliding movement of the contact probe 4 therein.
In fact, it should be remembered that the correct operation of a testing head is basically linked to two parameters: the vertical movement, or overtravel, of the contact probes and the horizontal movement, or scrub, of the contact tips of such probes.
These characteristics are therefore evaluated and calibrated in the manufacturing step of a testing head, the good electrical connection between the probes and the device to be tested being so guaranteed.
It is also possible to realize a testing head having contact probes protruding from a support, usually made of a ceramic material, possibly suitably preformed so as to ensure a proper bending thereof during the contact with the pads of a device to be tested. Such probes are further deformed when contacting the pads of the device to be tested.
The increase of the packing density of the probes required by the most modern technologies of integration on wafers involves problems of contact between adjacent probes, in particular during their deformation during the operation of the testing head.
In order to ensure a correct orientation of the probes, and in particular of their deformed sections, and therefore also of their deformation, it is known to realize contact probes having non-circular section, and in particular rectangular, and testing heads with guides having respective guide holes in turn having non-circular section, in particular rectangular, keeping the contact probes in position during their contact with the contact pads of the device to be tested and their consequent further deformation.
It is convenient to emphasize that the end portions of contact probes 4, at the contact head and contact tip 4A and 4B and in particular comprising the portions of the probes that are apt to slide in the guide holes 2A and 3A, are normally made so as to be tilted with respect to the axes of these holes (usually orthogonal to a plane defined by the device to be tested), in order to ensure the desired scrub on the contact pads.
The inclination of the end portions of the contact probes with respect to the axes of the guide holes then creates one or more points of contact between probes and holes, so as to realize an at least partial retention of the probes inside the holes.
It happens, however, that the retention of the probes, and in particular of their end portions, inside the guide holes is sometimes excessive, which limits the freedom of sliding of the probes themselves and affecting the proper operation of the testing head as a whole. In extreme conditions, the contact probes can “get stuck” inside the guide holes, completely stopping any operation of the testing head and leading to the need to replace it.
To ensure a correct sliding of the probe inside a relative guide hole as well as a proper hold in position of the probes inside the guides, while minimizing the risk that the probes get stuck and thus the need to replace the head, it is known to coat the end portions of the contact probes 4 with layers of a conductive material having a greater hardness than that of the conductive material forming the rest of the contact probe.
In particular, the coating layers extend in correspondence of a terminal portion of the respective end portion, from a tip up to the full height of the respective guide hole.
However, conductive materials with high hardness have also a marked fragility and can be made only in the form of films of reduced thickness, for example between 0.01 microns and 5 microns.
U.S. patent application published under No. US 2012/0286816, teaches to use conductive materials with high hardness for producing thin plates protruding from the body of the contact probes at their end portions. These plates, which are very resistant and able to penetrate a possible oxide layer which covers the contact pads, also allow in particular to make contact with the so-called bump, i.e. of conductive elements protruding from a device to be tested as contact portions, as it happens in other embodiments for the contact pads.
In some cases, a central portion of the contact probe is also coated with a layer of insulating material, such as parylene, apt to improve the electrical insulation of the probes, in particular avoiding short circuits in the event of accidental contacts between adjacent contact probes.
Noble metals coatings, in particular palladium-based noble metals coatings, of the end portions of the contact probes are also used to improve the contact of the end portions with respective contact pads, the materials forming the contact probes having in fact contact problems, in particular by varying the operating temperatures of the testing head including the contact probes.